In order to transmit video data more efficiently than by use of analog techniques, various forms of digital video signal compression systems have been proposed. The majority of these systems process sampled video signals on a block by block basis wherein a block includes a matrix of 8 pixels in each of 8 lines. A particular one of these systems is evolving into a standard which is known as "MPEG". Most current video signal compression processes tend to be lossy, that is, images reproduced from lossy compressed data represent less than the original.
One of the problems exhibited by reproduced video images, such as in the MPEG1 OR MPEG2 processes, is the visibility of contours in relatively flat field (smooth) dark images. That is, in darker flat field images, separate regions of different intensity which are separated by well defined contours are perceived. These contours are manifested by distinctive image intensity steps. The intensity differences between neighboring contoured regions may be small, however, the contours may be highly visible and highly objectionable under certain viewing conditions. Under normal viewing conditions, the contouring tends not to be perceptible. However, in low ambient light conditions (e.g., when viewing television in a darkened room) or when the image is reproduced with high contrast/brightness adjustments, the contouring is highly visible.
A similar contouring is exhibited by images which are reproduced by simple analog-to-digital-to-analog conversion when the conversion process is performed with insufficient quantizing resolution. If an image is quantized with, for example, eight bits, no contouring is apparent in reproduced images. However, if a video signal is quantized with eight bits and then compressed via a lossy process, and subsequently decompressed and reproduced, contouring will be apparent in darker flat field images.
The present inventor has determined that the main reason contouring is not perceived in the original digitized source image is due to the existence of dithering noise. Consider an analog video signal which is represented by a slowly changing ramp function traversing the values from n-1 to n+1. Respective consecutive values are captured and digitized. If the sampling instants occurred exactly at the instants the ramp exhibited the values n-1, n, n+1, the resulting signal would produce a contoured image. However, the sampling instants normally occur at instants the signal is between values, and probabilisticly, the conversion process may produce either of the values creating a signal representing "gray" areas, These gray areas tend to exhibit a low level noise, but this noise blurs or smoothes any contouring effects. Similarly, if the original signal contains random noise, the noise will provide low level interference of the conversion process, which will also produce a signal that tends to blur or smooth contours.
In image areas where the dithering noise plays a role in preventing contouring, any processing in between the video signal source and the signal destination, which intentionally or unintentionally cleans up or reduces the noise, will result in a degree of unintentional contouring. This may happen in a video encoder (compressor) as a result of bit-rate control. For example, in an MPEG encoder, blocks representing relatively smooth areas may be forced to a condition of all zero valued AC coefficient residues (the so called "no-code" condition), or the compression quantization may result in all zero valued AC coefficient residues. In addition, small deviations of the DC coefficient residues may be removed by compression quantizing, which deviations would otherwise tend to blur or smooth contours in reproduced images.
MPEG encoding is performed using a limited dynamic range. A clamping condition occurs if the values of quantized coefficients exceeds the dynamic range, which clamping results in highly undesirable visual artifacts. MPEG implementations generally check for a clamping condition, and on the occurrence of such, restart the quantization process with coarser quantization parameters, in an iterative manner, until the clamping condition has been eliminated. The coarser quantization parameters utilized to cure clamping removes much if not all dithering noise effects, giving rise to blockiness and contouring.